A post-deflection electrostatic acceleration and scan expansion electron lens system incorporated in, for example, a cathode-ray tube (CRT) typically performs two distinct functions. First, the lens system increases the angle of electron beam deflection produced by the deflection structures of the CRT to scan the beam over an area of a desired size on the display screen. Second, the lens system accelerates the beam electrons. The acceleration of the beam electrons is characterized as being of either positive or negative sign, with positive acceleration and negative acceleration indicating beam electron acceleration directed toward and away from, respectively, the display screen of the CRT. Positive acceleration increases the energy of the beam electrons and thereby produces a brighter image on the display screen. Negative acceleration repels low-energy and secondary emission electrons away from the display screen and thereby reduces the number of spurious light patterns present in the image.
One type of acceleration and scan expansion lens system makes use of a quadrupole lens of the Klemperer-type, which comprises a pair of adjacent, cylindrical electrode elements. A CRT employing such a lens system typically includes separate deflection structures for deflecting horizontally in the X-direction and vertically in the Y-direction an electron beam traveling toward a display screen in the Z-direction of a three-dimensional Cartesian coordinate system. The horizontal and vertical deflection structures typically have different lengths as measured in the Z-direction, are separated from the display screen by different amounts, and operate in response to signals provided by respective horizontal and vertical deflection signal amplifiers having different gain characteristics. These differences provide a CRT with different deflection sensitivities in the horizontal and vertical directions.
The quadrupole scan expansion lens converges and diverges the beam electrons in different ones of the X-Z and Y-Z planes. The particular planes of convergence and divergence are determined by the arrangement of, and the relative magnitudes of voltages applied to, the quadrupole lens electrodes. In the convergence plane of the quadrupole lens, scan expansion results from focusing the beam electrons to a point at a location near the lens and then allowing the beam to diverge from that point, thereby employing over-convergence to expand the scan of the electron beam. It is very difficult, therefore, to design a Klemperer-type quadrupole lens that would provide the preferred scan expansion in both the X-Z and Y-Z planes. As a consequence, such a lens typically matches the deflection sensitivities of the CRT in only one of the X-Z and Y-Z planes, thereby providing a beam image spot size that is optimized in only one of the X-Z and Y-Z planes.
A quadrupole lens converges and diverges beam electrons in different ones of the X-Z and Y-Z planes by generating substantial field variations over relatively short distances. As a consequence of these field variations, the scan expansion performance of a quadrupole lens is dramatically altered by slight variations in the positioning of the lens electrode elements. Such positioning variations would include, for example, those that occur during production-type manufacturing.